This invention relates to burn-in and test of microelectronic devices, specifically to contact assemblies used for connecting electrical signals to integrated circuits during burn-in and test of individual chips and of full wafers.
Microelectronic devices are subjected to a series of test procedures during the manufacturing process in order to verify functionality and reliability. The testing procedures conventionally include wafer probe testing, in which microelectronic device chips are tested to determine operation of each chip before it is diced from the wafer and packaged. Probe cards built of long cantilever wires are used to test one or several chips at a time while on the wafer.
Typically, not all chips on a wafer are found to be operable in the wafer probe test, resulting in a yield of less than 100% good devices. The wafer is diced into individual chips, and the good chips are then assembled into packages. The packaged devices are dynamically burned-in by loading them into sockets on burn-in boards and electrically operating them at a temperature of from 125xc2x0 C. to 150xc2x0 C. for a burn-in period of 8 to 72 hours in order to induce any defective devices to fail. Burn-in accelerates failure mechanisms that cause infant mortality or early failure of the devices, and allows these defective devices to be weeded out by a functional electrical test before they are used commercially.
A full functional test is done on packaged devices, which are operated at various speeds in order to categorize each by maximum speed of operation. Testing discrete packaged devices also permits elimination of any devices that failed during the burn-in process. Burn-in and test of packaged devices is accomplished by means of sockets specially suited to the burn-in conditions and to high speed testing respectively. Conventional manufacturing processes are expensive and time consuming because of a repeated handling and testing of individual discrete devices through a lengthy set of steps that adds weeks to the total manufacturing time for the device.
A considerable advantage in cost and in process time can be obtained by burn-in and test of the wafer before it is diced into discrete devices. Additional savings can be obtained by fabricating chip size packages on each device on a wafer before the wafer is diced into discrete devices. A considerable effort has been expended by the semiconductor industry to develop effective methods for wafer level burn-in and test in order to gain benefits of a greatly simplified and shortened process for manufacturing microelectronic devices. In order to reap these benefits, it is necessary to provide means to burn-in and speed test chips before they are diced from the wafer into individual discrete devices.
Conventional cantilever wire probes are not suited to burn-in and speed test of devices on the wafer. Cantilever wire probes are too long and costly to allow simultaneous contact to all of the devices on a wafer, as required for simultaneous burn-in of all of the devices on the wafer. In addition, long cantilever wire probes are not suitable for functional testing of high-speed devices because of a high self and mutual inductance of the long, parallel wires comprising the probes.
A small, high-performance probe that can be made at low cost is required for practical application of wafer burn-in and test procedures. To be useful for wafer burn-in and test, the probes must reliably contact all of the pads on the devices under test while they are on the undiced wafer. Probes for contacting the wafer must provide electrical contact to pads on devices where the pads vary in height on the surface of the wafer. In addition, the probes must break through any oxide layers on the surface of the contact pads in order to make a reliable electrical contact to each pad. Many approaches have been tried to provide a cost-effective and reliable means to probe wafers for burn-in and test, without complete success.
A number of attempts have been tried to provide small, vertically compliant probes for contacting reliably the pads on devices on a wafer. According to the invention represented by U.S. Pat. No. 4,189,825, to David R. Robillard and Robert L. Michaels, a cantilever probe is provided for testing integrated circuit devices. In FIG. 1, cantilever 28 supports sharp tips 26 above aluminum contact pads 24 on a chip 23. A compliant member 25 is urged downward to move tips 26 into contact with pads 24. An aluminum oxide layer on pad 24 is broken by sharp tip 26 in order to make electrical contact between tip 24 and the aluminum metal of pad 24. The rigidity of small cantilever beams is generally insufficient to apply the force to a tip that is necessary to cause it to break through an aluminum oxide layer on a contact pad, without an external means of applying force to the cantilever. Cantilever beams of glass, silicon, ceramic material, and tungsten have been tried in various configurations, without success in providing burn-in probes of sufficient force and flexibility.
A flexible membrane probe shown in FIG. 2A is described in Flexible Contact Probe, IBM Technical Disclosure Bulletin, October 1972, page 1513. A flexible dielectric film 32 includes terminals 33 that are suited to making electrical contact with pads on integrated circuits. Terminals 33 are connected to test electronics by means of flexible wires 34 attached to contact pads 35 on terminals 33. Probes fabricated on a flexible polyimide sheet were described in the Proceedings of the IEEE International Test Conference (1988) by Leslie et al. The flexible sheet allows a limited amount of vertical motion to accommodate variations in height of bond pads on integrated circuits on a wafer under test. Membrane probes such as that described by Leslie et al provide connections to integrated circuit chips for high performance testing. However, dimensional stability of the membrane is not sufficient to allow contacts to pads on a full wafer during a burn-in temperature cycle.
Fabrication of the contacts on a thin silicon dioxide membrane is shown in FIG. 2B as described in U.S. Pat. No. 5,225,771 by Glenn J. Leedy. A silicon dioxide membrane 40 has better dimensional stability than polyimide, thereby somewhat ameliorating the dimensional stability problem of mating contacts to pads on a wafer under test. Probe tips 41 are connected by vias 44 through membrane 40 to circuit traces 45 that are linked to an additional layer of circuitry 42 above a dielectric film 43. Limited vertical compliance of the test probes on silicon dioxide membrane 40 renders use of probe arrays unreliable for use in burn-in of devices on a semiconductor wafer.
Fabrication of an array of burn-in probes on a semiconductor wafer is described in U.S. Pat. No. 4,585,991, as illustrated in FIGS. 3A and 3B showing a top plan view and a sectional view respectively. Probe 51 is a pyramid attached to semiconductor wafer substrate 52 by arms 54. Material 53 is removed from the semiconductor wafer 52 in order to isolate mechanically the probe 51. A probes as in FIG. 3A provides a limited vertical movement but do not allow space on the substrate for wiring needed to connect an array of probes to test electronics required for dynamic burn-in.
An approach to providing flexible probes to device contact pads involves the use of flexible wires or posts to connect test circuitry to pads on a chip. A flexible probe shown in FIG. 4A is described in U.S. Pat. No. 5,977,787 by Gobina Das et al. Probe 60 is a buckling beam, generally described in U.S. Pat. No. 3,806,801 by Ronald Bove. Probe 60 is adapted for use in burn-in of devices on a wafer. Probe 60 is held by guides 61 and 62 that have a coefficient of expansion similar to that of the wafer being tested. Probe 60 is offset by a small distance 63 to provide a definite modality of deflection. Although buckling beams are well suited to testing individual integrated circuit chips, they are too expensive to be used for wafer burn-in where thousands of contacts are required. Further, electrical performance of buckling beam probes is limited because of the length required for adequate flexure of the beam.
Another approach using flexible posts is shown in FIG. 4B as disclosed in U.S. Pat. No. 5,513,430 by Arnold W. Yanof and William Dauksher. FIG. 4b shows flexible probes in the form of posts 66 that are able to bend in response to force on probe tip 67. Posts 66 are formed at an angle to a substrate 69 in order to allow them to flex vertically in response to a force on tip 67 from mating contact pads. Posts 66 have a taper 65 from the base terminal 68 to tip 67 in order to facilitate flexure.
Yet another approach using flexible wires and posts is shown in FIG. 4C as disclosed in U.S. Pat. No. 5,878,486 by Benjamin N. Eldridge, et al. The probe shown in FIG. 4C comprises a probe tip 72 on a spring wire 71 that is bent to a specific shape in order to facilitate flexure. Wire 71 is joined to substrate 74 by a conventional wire bond 73. Probes of the type shown in FIG. 4C require a long spring length to achieve the contact force and compliancy needed for wafer burn-in. Additionally, such probes that use individual wires are too expensive for use in wafer burn-in where many thousands of probes are required for each wafer.
Further approaches to providing flexible probes involve the use of compliant layers interposed between a test head and a device being tested, such that terminals on the test head are electrically connected to mating contact pads on the device. The electrical connector described in U.S. Pat. No. 3,795,037 by Willem Luttmer utilizes flexible conductors embedded in an elastomer material to make connections between mating pairs of conductive lands that are pressed into contact with the top and bottom surfaces of the electrical connector. Many variations of flexible conductors including slanted wires, conductive filled polymers, plated posts and other conductive means in elastomeric material in order to form compliant interposer layers.
The approaches listed above and other attempts have been unsuccessful in providing a high performance probe that allows economical burn-in and speed test of microelectronic devices on a wafer before the wafer is diced into discrete chips.
In accordance with the present invention, a single-sided compliant probe is provided that includes a conductive tip, which is positioned on a supporting substrate in a manner that allows a tip on the probe to move flexibly with respect to the supporting substrate, while in close proximity to adjacent probes in an array. The probe tip moves vertically in response to the force of a mating contact pad as it is mechanically biased against the tip. Mechanical compliance of the probe allows electrical contact to be made reliably between the probe and a corresponding contact pad on a microelectronic device, where the mechanical compliance accommodates variations in height of the contact pad.
The present invention is useful for making electrical connection to contact pads on microelectronic devices on an undiced wafer in order to burn-in the devices before they are diced into separate chips. Compliant probes according to the invention allow reliable electrical connections to be made simultaneously to all of the contact pads arrayed on the surface of a wafer so that microelectronic devices on the wafer can be burned-in economically. Mechanical compliance of probes in the fixture accommodates variations in height of the contact pads and in the probe tips such that each probe tip remains in contact with its mating contact pad throughout the temperature cycle of the burn-in process.
The present invention also provides an element of an electrical probe card that allows high speed testing of unpackaged microelectronic devices. Small, single-sided compliant probes as taught in this disclosure are used to make temporary connections to corresponding pads on a device in order to apply electrical test signals to that device and to measure electrical signals from that device. The small size of the compliant probe allows high speed electrical signals to be passed to and from the device without losses due to excessive inductance or capacitance associated with wire probes as used in the prior art.
Small, compliant probes as taught in this disclosure are used to make reliable electrical connections to contacts on the device, where the contacts are arranged in an area array. Mechanical compliance allows the tip of each probe to maintain electrical contact with a mating contact on the device notwithstanding variations in the height of contacts on the device both at room temperature and at the operating temperature range of the device.
The present invention also provides a small socket for connecting integrated circuit chips to electrical circuits for purposes of burn-in, test and operation of the chip. The small size of each probe contact in the socket allows high-speed operation of a chip mounted in the socket. Mechanical compliance of the probes as taught in this disclosure enables reliable electrical connections to be made to a rigid chip with minimal or no packaging. Compliant probes according to the present invention allow construction of small, economical sockets for chip scale packages and for flip-chips.
The probe disclosed herein is significantly improved over conventional cantilever probes in that it provides a greater range of compliant motion of the probe tip for any given probe force and probe size. A conventional cantilever probe is limited by the range of motion it provides in response to a given force the elastic limit of the probe material is reached. The maximum mechanical stress in cantilever probes is concentrated on the surface of the cantilever material at the point of flexure. The invention provides a greater range of motion for a given spring material and probe force before it reaches the elastic limit of that material.
The invention increases manufacturing efficiency for microelectronic devices by providing test and burn-in functions reliably at the wafer level, while at the same time reducing the size of the test fixture. The mechanically compliant probe provides a large range of motion relative to the size of the probe. This range of motion is important in making connections to a device with contact pads that are not substantially in the same plane. The compliant probe tip moves flexibly to accommodate differences in the height of mating contact pads while maintaining sufficient force of the probe tip on the contact pad to assure reliable electrical contact therebetween.
A probe tip is disposed on an elongated thin strip of material that is supported rigidly by a rigid post at one end and slidably by a protrusion disposed at a distance from the first end, wherein the tip is located at a predetermined distance from a center line connecting the centers of the rigid post and the protrusion. The probe tip thus supported is able to move compliantly in a vertical direction by torsional and bending flexure of the thin strip of material.
The invention is able to increase manufacturing efficiency for microelectronic devices by performing test and burn-in functions reliably at the wafer level, while at the same time reducing the size of the test fixture.